Stress control of semiconductor microstructures for thin film growth

ABSTRACT

A suspended semiconductor film is anchored to a substrate at at least two opposed anchor positions, and film segments are deposited on the semiconductor film adjacent to one or more of the anchor positions to apply either tensile or compressive stress to the semiconductor film between the film segments. A crystalline silicon film may be anchored to the substrate and have tensile stress applied thereto to reduce the lattice mismatch between the silicon and a silicon-germanium layer deposited onto the silicon film. By controlling the level of stress in the silicon film, the size, density and distribution of quantum dots formed in a high germanium content silicon-germanium film deposited on the silicon film can be controlled.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of provisional applicationNo. 60/333,331, filed Nov. 26, 2001, the disclosure of which isincorporated herein by reference.

REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with United States government supportawarded by the following agency: NSF 0079983. The United Statesgovernment has certain rights in this invention.

FIELD OF THE INVENTION

[0003] This invention pertains generally to the field of semiconductorprocessing and particularly to the growth of thin films of semiconductorin a controlled manner.

BACKGROUND OF THE INVENTION

[0004] Significant improvements in the functionality of integratedcircuits may be obtained by integrating different materials withspecialized properties onto a base semiconductor such as crystallinesilicon. For example, semiconductor structures incorporating thin filmsof gallium nitride, germanium, germanium-silicon, etc. on a silicon basewould enable the development of transistors with integrated opticscapabilities or the capability of operation at much higher frequenciesthan presently possible. A major obstacle to growing thin films of onesemiconductor material on another (such as germanium on silicon) is thelattice mismatch and consequent strain-induced film morphology. Thisobstacle controls and limits the film morphology and, in turn, theelectrical and optical properties of the film.

[0005] Strain induced thin film morphology limitations are encountered,for example, in the heteroepitaxial growth of silicon-germanium oncrystalline silicon substrates. It is found that heteroepitaxial growthof Si_(1−x)Ge_(x) with x>0.2 typically results in the formation ofthree-dimensional islands which can act as quantum dots (QDs) becausethey localize charge. These coherently strained QDs form naturally as astrain reduction mechanism for the film. If x is less than 0.2, astrained film is formed which does not have the 3D islands.Heterojunction bipolar transistors (HBTs) have been made using suchdefect-free epitaxial films of silicon-germanium and have showndramatically improved performance relative to silicon HBTs. However, itwould be desirable to be able to increase the germanium concentration insuch films beyond that which has been possible in the prior art becauseof the occurrence of the quantum dot islands at higher germaniumconcentrations. In addition, the thickness of a heteroepitaxialsilicon-germanium film of a given germanium concentration is limited bythe critical thickness at which dislocations form because of the 4.2%lattice mismatch. While it would be desirable to be able to producethicker defect-free films for many applications such as HBTs, forcertain applications, the quantum dots that are formed in strained filmsare desirable because of potential applications in quantum computationand communication, light detectors, and lasers. It would be preferablefor many of these applications that the arrangement of quantum dots beregular, rather than random, and with a narrow quantum dot sizedistribution.

SUMMARY OF THE INVENTION

[0006] The micromachined structures of the present invention provide aselected surface stress level in a semiconductor film, such as silicon,that allows the growth of an epitaxial film on the semiconductor film ina controlled manner to result in desired properties. Such structures canbe produced by lithography and scaled to sub-micron dimensions. Parallelbatch fabrication of multiple devices on a silicon wafer can be carriedout with subsequent dicing of the structures after fabrication oftransistors or other devices.

[0007] In the present invention, a biaxial or uniaxial tensile orcompressive stress is applied to a suspended semiconductor film (e.g.,crystalline silicon) and a heteroepitaxial film of semiconductormaterial is grown on the stressed semiconductor film. The stress in thebase semiconductor film is introduced by utilizing thin film segmentsdeposited on the semiconductor film which have strain with respect tothe semiconductor film as deposited, applying either tensile orcompressive stress to the semiconductor film. Utilizing the inducedstress on the semiconductor film, the natural lattice mismatch strainand/or thermal expansion strain can be enhanced or countered, asdesired, allowing growth morphologies to be controlled to allowapplications such as the production of specific arrays of quantum dots,high germanium concentration films, and arrays of quantum dots withcontrolled size distributions.

[0008] The semiconductor microstructures of the invention include asuspended semiconductor film anchored to a substrate at at least twoopposed anchor positions, and strain inducing thin film segmentsdeposited on the semiconductor film adjacent to the anchor positions toapply either compressive or tensile stress to the semiconductor filmbetween the film segments. Crystalline silicon may be utilized as thesemiconductor film, although it is understood that other semiconductors(e.g., germanium, gallium arsenide, etc.), or other forms of thesemiconductor (e.g., polycrystalline silicon) may constitute thesemiconductor film. For silicon thin films, the film segments maycomprise layers of, e.g., silicon dioxide and silicon nitride which areparticularly suited to apply tensile stress to the semiconductor film.The semiconductor film may be formed as a beam which is anchored to thesubstrate at two opposed positions and is suspended from the substratebetween the two opposed anchor positions. The semiconductor film mayalso be formed with arms anchored to the substrate at multiple pairs ofopposed anchor positions to apply stress in multiple directions to acentral portion of the semiconductor film. A layer of material such assilicon-germanium may be deposited on the central region of thesemiconductor film, with the characteristics of the deposited layeraffected by the stress in the underlying semiconductor film. Forexample, in accordance with the invention, the number of quantum dots insilicon-germanium deposited on a silicon semiconductor film is inverselyrelated to the tensile stress imposed on the underlying semiconductorfilm.

[0009] The semiconductor microstructures in accordance with theinvention may be formed by providing a semiconductor structure includingat least a layer of semiconductor film over a sacrificial layer, withthe semiconductor film secured to a substrate. A film of material isthen deposited over the semiconductor film that is in tensile orcompressive strain with respect to the semiconductor film. The depositedfilm is patterned to leave opposed segments spaced from each other by acentral region of the semiconductor film. The semiconductor film is thenpatterned and the sacrificial layer is removed beneath the patternedsemiconductor film to leave a semiconductor film section anchored to thesubstrate at opposed anchor positions. The film segments remain on thesemiconductor film adjacent to the anchor positions and spaced from eachother by the central region of the suspended semiconductor film suchthat the film segments apply a tensile or compressive stress to thesuspended semiconductor film.

[0010] Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings:

[0012]FIG. 1 is a plan view of multiple semiconductor microstructures inaccordance with the invention having a semiconductor film anchored tothe substrate at two opposed anchor positions.

[0013]FIG. 2 is a more detailed plan view of a portion of a suspendedsemiconductor film having notches formed therein to provide stressconcentration.

[0014]FIG. 3 is a photomicrograph illustrating formation of quantum dotsin a silicon-germanium layer deposited on the microstructure of FIG. 2at a low stress position.

[0015]FIG. 4 is a photomicrograph as in FIG. 3 illustrating theformation of quantum dots in the deposited layer surface at anintermediate stress position.

[0016]FIG. 5 is a photomicrograph as in FIG. 3 illustrating the layersurface at a highly strained position.

[0017]FIG. 6 is an illustrative side view of a silicon-on-insulatorstructure that may be utilized in forming semiconductor microstructuresin accordance with the invention.

[0018]FIG. 7 is a view as in FIG. 6 at a further processing step.

[0019]FIG. 8 is a view as in FIG. 7 after a further processing step.

[0020]FIG. 9 is a view as in FIG. 8 after a further processing step.

[0021]FIG. 10 is a view as in FIG. 9 after a further processing step.

[0022]FIG. 11 is a view as in FIG. 10 after a final processing stepleaving a suspended semiconductor film with film segments thereon.

[0023]FIG. 12 is a view similar to that of FIG. 9 after an alternativeprocessing step.

[0024]FIG. 13 is a view as in FIG. 12 after a further processing step.

[0025]FIG. 14 is a view as in FIG. 13 after a final processing stepleaving the suspended semiconductor film with film segments thereon.

[0026]FIG. 15 is a photomicrograph showing a large quantum dot formed inthe deposited layer at a high stress area of the semiconductor structureas in FIG. 2.

[0027]FIG. 16 is a photomicrograph illustrating a perspective view of alarge quantum dot as in FIG. 15.

[0028]FIG. 17 is a plan view of another semiconductor microstructure inaccordance with the invention having a semiconductor film with armsanchored to the substrate at two pairs of opposed anchor positions thatare diagonally oriented with respect to each other.

[0029]FIG. 18 is a plan view of a semiconductor microstructure having asemiconductor film with arms anchored to the substrate at four pairs ofopposed anchor positions.

[0030]FIG. 19 is a plan view of another semiconductor microstructurewith a semiconductor film also having arms anchored to the substrate atfour pairs of opposed anchor positions.

DETAILED DESCRIPTION OF THE INVENTION

[0031] With reference to FIG. 1, an example of the invention in whichthe semiconductor film is formed as a beam anchored at two ends to forma bridge is illustrated generally at 30. A plurality of differentsemiconductor film bridge beams 31 of various lengths and configurationsare shown. Each of the semiconductor film beams 31 is anchored to asubstrate 32 at opposed anchor positions 33 and 34. The anchored beams31 are suspended over an opening or depression 35 in the substrate. Thinfilm segments 38 and 39 are deposited on the semiconductor film beams 31adjacent to the anchor positions 33 and 34, respectively. As illustratedin FIG. 1, the thin film segments 38 and 39 may be deposited both over aportion of the suspended beam 31 as well as over the surface of thesubstrate 32. The film segments 38 and 39 are spaced from each other toleave a central position 40 of each of the beams 31 that is not coveredby the films 38 and 39 and which is available to be used for otherpurposes, e.g., formation of devices thereon, and particularly for thedeposit of a heteroepitaxial film of another semiconductor. Thedeposited film segments 38 and 39 have a strain as deposited on thesemiconductor film beams 31 to apply either compressive or tensilestress to the beams 31. Commonly, the film segments 38 and 39 will bedeposited to provide tensile strain, thereby applying tensile stress tothe thin film beams 31, with the tensile stress induced in a directionparallel to the length of the beams 31. The stress in the beams may beconcentrated by utilizing notches or cutouts as illustrated at 41 inFIG. 1 or by forming openings (not shown) in the beams.

[0032]FIG. 2 illustrates an example of the beam structure of FIG. 1having notches 41 therein for concentration of stress. An exemplarystructure of this type was formed, as described further below, toprovide a crystalline silicon thin film bridge 31 and deposited filmsegments 38 and 39 formed of layers of silicon dioxide and siliconnitride. A layer of silicon-germanium was then deposited on the centralportion 40 of the beam and annealed. Atomic force microscope imagestaken from the position marked A, B and C in FIG. 2 are shown in FIG. 3,FIG. 4, and FIG. 5, respectively. Due to the shape of the bridgestructure, the highest strain occurs near the notches 41 at the positionC and the lowest strain occurs near the deposited film segments 38 and39, such as at the position marked A in FIG. 2. FIG. 3, taken from theposition marked A in FIG. 2, shows a quantum dot morphology similar tothat which would be seen for silicon-germanium growth on an unstrainedsilicon substrate. FIG. 4, at an area of greater tensile stress in thebeam 31, shows a significant decrease in the density of quantum dots,indicating that coarsening throughout the anneal period has resulted ina lower density of small quantum dots. In the image of FIG. 5, from thehighest stress area of the beam, the density of small quantum dots isextremely low (none are visible in the scan area; the black spot is aminor defect in the silicon buffer layer).

[0033] An exemplary process for forming the stressed thin filmstructures of the invention is illustrated with respect to FIGS. 6-11.In this exemplary process, the device fabrication begins with acommercial silicon-on-insulator wafer 45 composed of a base substratelayer of crystalline silicon 46, a silicon dioxide layer 47, and a topcrystalline silicon layer 48, (e.g., a 4 inch<100>SOI(Si/SiO₂/Si=10/1/550 μm). The wafer 45 is initially oxidized with a 0.8μm thermal oxide (at 1050° C.) coating 50 as shown in FIG. 7. The oxidecoating 50 serves to protect silicon areas where quantum dot growthoccurs after micromechanical or microelectronic structures arefabricated. The oxide layer on the front side of the wafer is thenetched to form openings 51 and to leave a central oxide layer 52 atpositions at which the thin film segments are to be formed, as shown inFIG. 8, and a layer 53 of silicon nitride is then deposited over thewafer including the open areas 51, as shown in FIG. 9. The front sidesilicon nitride layers are then removed by deep reactive ion etching(RIE (UW-STS)) to open the areas 51 leaving the silicon dioxide layer 52and the silicon nitride layer 53 as the thin film segments over thesilicon layer 48, as shown in FIG. 10. RIE etching of the silicon layer48 is followed by etching of the buried oxide layer 47, acting as asacrificial layer, e.g., by HF:HCl (1:1), as illustrated in FIG. 11, torelease a suspended silicon beam 31 formed from the silicon layer 48.The overlying film segments composed of the layers 52 and 53 havebuilt-in strain to stress the suspended beam 31 in, e.g., tensile,stress. As illustrated in FIG. 11, the thin film beam 31 is suspendedabove a surface 56 of the base substrate 46 in the area of the opening58 which was opened by the etching processes. It is understood that thebeam 31 is anchored at opposed anchor positions to the silicon substrate46 by the oxide layer 47 and the silicon layer 48 (which, in this case,is integral with the silicon beam 31).

[0034] An alternative procedure for freeing the beam is illustrated withrespect to FIGS. 12-14. With the front side silicon nitride layerremoved, as was shown in FIG. 10, an RIE etching is carried out throughthe oxide layer 48 to isolate the thin film beam 31 on the oxide layer47, as shown in FIG. 12. The backside silicon nitride layer 53 is thenremoved, e.g., by RIE, followed by a backside etching of the substratesilicon layer 46 that terminates on the oxide layer 47 to leave anopening 60 in the backside of the wafer, as shown in FIG. 13. The oxidelayer 47 in the exposed area 60 is then etched away to release the beam31 with the thin film segments 33 or 34, composed of the layers 52 and53, over portions of the beam 31.

[0035] After patterning and releasing of the stressed suspended siliconfilm beams 31, a thin film of another semiconductor may be deposited onthe stressed films 31. For example, silicon-germanium films may be grownon the stressed films 31. As an example, silicon-germanium films atvarious silicon to germanium ratios were formed on the film beams 31 ofFIG. 1. After these beams were patterned and released, they werechemically cleaned and loaded into a UHV Molecular Beam Epitaxy (MBE)chamber where growths of Si Ge were performed between 450° C. and 550°C. At a high flux rate of Ge, the film growth is kinetically limited,which causes a high density of small quantum dots (less than 30 nm) toform, regardless of the stress on the beams 31. By subjecting thesamples to a 30 minute post-growth anneal at the growth temperature,however, clear differences in morphology on the strained and unstrainedareas of the substrate were found. FIGS. 3-5, discussed above,illustrate a density gradient of small quantum dots which is inverselyproportional to the underlying substrate strain. Following a 30 minuteanneal at the growth temperature of 550° C., a few extremely largeislands (about 0.5 μm) were found, as illustrated in FIGS. 15 and 16. Asshown in FIG. 15, small quantum dot islands were seen surrounding thelarge island denoted at 62 in FIGS. 15 and 16. In the highly strainedareas of the silicon beam, the densities of these large islands is asmuch as 50 times greater than in the unstrained areas. The lower densityof small quantum dots seen in FIGS. 4 and 5 can be attributed to therelatively high density of nearby large islands.

[0036] The semiconductor film can be anchored to the substrate at morethan two positions so that the film can be stressed in more than onedirection. FIG. 17 illustrates a semiconductor film 69 formed as a crosswith two pairs of arms 70 which are anchored to the substrate 71 at twopairs of anchor positions 72 and 73, which are diagonally oriented withrespect to each other. The cross-shaped film 69 is suspended above anopening or depression 75 in the substrate 71. Film segments 76 aredeposited on the arms 70, as described above, adjacent to the opposedpairs of anchor positions 72, and film segments 78 are deposited overthe arms 70 of the semiconductor film 69 at positions adjacent to theanchor positions 73. The film segments 76 and 78 strain thesemiconductor film 69 in diagonally opposed directions so that thecenter portion 80 of the semiconductor film is stressed in twoperpendicular directions. As shown in FIG. 18, a semiconductor film 85may be anchored to a substrate 86 at four pairs of anchor positions 88,89, 90, and 91. A series of arms 94 extend away from a center portion 95of the semiconductor film 85 to be anchored to the substrate at theanchor positions 88-91. Portions of the arms 94 have film segments 98deposited thereon to stress the arms and thereby provide stressextending in four symmetrically distributed directions to the centerportion 95 of the semiconductor film 85. FIG. 19 illustrates a variationon the structure of FIG. 18 in which the film segments 98 essentiallyentirely cover the arms 94 of the suspended semiconductor film, againproviding symmetrical stress to the center portion 95 of thesemiconductor film, but with greater stress levels than are imposed withthe structure of FIG. 18.

[0037] It is understood that the invention is not confined to theparticular embodiments set forth herein as illustrative, but embracesall such forms thereof as come within the scope of the following claims.

What is claimed is:
 1. A semiconductor microstructure comprising: (a) asubstrate; (b) a semiconductor film anchored to the substrate at atleast two opposed anchor positions and suspended between the anchorpositions; (c) a film segment of material deposited on the semiconductorfilm adjacent to at least one anchor position, the film segment having acompressive or tensile strain as deposited on the semiconductor film toapply a compressive or tensile stress to the suspended semiconductorfilm between the film segments.
 2. The microstructure of claim 1 whereinthe substrate and semiconductor film are formed of crystalline siliconand the film segments are formed of at least a layer of silicon nitride.3. The microstructure of claim 1 wherein the film segment comprises alayer of silicon dioxide deposited on the semiconductor film and a layerof silicon nitride deposited on the silicon dioxide layer.
 4. Themicrostructures of claim 1 wherein a film segment is deposited on thesemiconductor film adjacent to each anchor position such that filmsegments are adjacent to opposed anchor positions and are spaced fromeach other by a section of the semiconductor film.
 5. The microstructureof claim 4 wherein the semiconductor film is formed as a beam anchoredto the substrate at two opposed positions and is suspended from thesubstrate between the two opposed anchor positions.
 6. Themicrostructure of claim 4 wherein the semiconductor film is anchored tothe substrate at a plurality of pairs of opposed anchor positions. 7.The microstructure of claim 6 wherein the semiconductor film is anchoredto the substrate at two opposed pairs of anchor positions.
 8. Themicrostructure of claim 6 wherein the semiconductor film is anchored tothe substrate at four opposed pairs of anchor positions.
 9. Themicrostructure of claim 4 wherein the semiconductor film is formed ofcrystalline silicon and the film segments at opposed anchor positionsare spaced from each other to define a central portion of thesemiconductor film, the film segments are in tensile strain as depositedand apply a tensile stress to the semiconductor film, and including alayer of silicon-germanium deposited on the central portion of thesemiconductor film.
 10. The microstructure of claim 9 wherein the layerof silicon-germanium has a pattern of quantum dot inclusions ofgermanium therein with a density related to the level of stress in thecentral region of the silicon semiconductor film.
 11. The microstructureof claim 4 wherein the semiconductor film has a central portion and aplurality of arms extending therefrom to opposed anchor positions atwhich the arms are anchored to the substrate, a film segment depositedon each arm adjacent the anchor position.
 12. A method of forming asemiconductor microstructure comprising: (a) providing a semiconductorstructure including at least a layer of semiconductor film over asacrificial layer, the semiconductor film secured to a substrate; (b)depositing a film of material over the semiconductor film that has atensile or compressive strain with respect to the semiconductor film;(c) patterning the deposited film to leave opposed segments spaced fromeach other by a central portion of the semiconductor film; and (d)patterning the semiconductor film and removing the sacrificial layerbeneath the patterned semiconductor film to leave a semiconductor filmsection anchored to the substrate at at least two opposed anchorpositions, with the film segments remaining on the semiconductor filmadjacent to the anchor positions and spaced from each other by thecentral position of the suspended semiconductor film such that the filmsegments apply a tensile or compressive stress to the suspendedsemiconductor film.
 13. The method of claim 12 wherein the semiconductorfilm comprises crystalline silicon and including the further step ofdepositing a layer of silicon-germanium over the central portion of thestressed semiconductor film.
 14. The method of claim 13 wherein thesilicon-germanium film is deposited by molecular beam epitaxy.
 15. Themethod of claim 13 wherein the film of material deposited on thesemiconductor film comprises a layer of silicon dioxide deposited on thesemiconductor film and a layer of silicon nitride deposited on thesilicon dioxide layer and wherein the deposited film is formed intensile strain with respect to the semiconductor film to apply tensilestress thereto.
 16. The method of claim 13 further including annealingthe microstructure with the silicon-germanium layer thereon.
 17. Asemiconductor microstructure comprising: (a) a substrate; (b) asemiconductor film formed of crystalline silicon anchored to thesubstrate at at least two opposed anchor positions and suspended betweenthe anchor positions; (c) a film segment of material deposited on thesemiconductor film adjacent each anchor position such that the filmsegments are adjacent to opposed anchor positions and are spaced fromeach other by a central portion of the semiconductor film, the filmsegments having a tensile strain as deposited on the semiconductor filmto apply a tensile stress to the suspended semiconductor film betweenthe film segments; and (d) a layer of silicon-germanium deposited on thecentral portion of the semiconductor film.
 18. The microstructure ofclaim 17 wherein the film segments are formed of at least a layer ofsilicon nitride.
 19. The microstructure of claim 17 wherein the filmsegments comprise a layer of silicon dioxide deposited on thesemiconductor film and a layer of silicon nitride deposited on thesilicon dioxide layer.
 20. The microstructure of claim 17 wherein thesemiconductor film is formed as a beam anchored to the substrate at twoopposed positions and is suspended from the substrate between the twoopposed anchor positions.
 21. The microstructure of claim 17 wherein thesemiconductor film is anchored to the substrate at a plurality of pairsof opposed anchor positions.
 22. The microstructure of claim 21 whereinthe semiconductor film is anchored to the substrate at two opposed pairsof anchor positions.
 23. The microstructure of claim 21 wherein thesemiconductor film is anchored to the substrate at four opposed pairs ofanchor positions.
 24. The microstructure of claim 17 wherein the layerof silicon-germanium has a pattern of quantum dot inclusions ofgermanium therein with a density related to the level of stress in thecentral region of the silicon semiconductor film.
 25. The microstructureof claim 17 wherein the semiconductor film has a plurality of armsextending therefrom to opposed anchor positions at which the arms areanchored to the substrate, a film segment deposited on each arm adjacentthe anchor position.
 26. The microstructure of claim 17 wherein layer ofsilicon-germanium is in the ratio Si_(1−x)Ge_(x) where x is greater than0.2, and wherein the tensile stress in the semiconductor film isselected such that regions of the silicon-germanium layer are free ofquantum dot inclusions.